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排序方式: 共有171条查询结果,搜索用时 31 毫秒
1.
Sanchez-sanz M; Blyth MG 《The Quarterly Journal of Mechanics and Applied Mathematics》2007,60(2):125-138
Unsteady, axisymmetric stagnation flow about a circular cylinderis examined when the far-field flow is a periodic function oftime with a fixed time average and an oscillatory part of prescribedamplitude and frequency. Solutions are computed for arbitraryvalues of the Reynolds number, quantifying the effects of surfacecurvature, and a frequency parameter based on the period ofthe far-field flow. It is found that solutions remain regularand periodic provided that the far-field amplitude lies belowa critical value. Above this value, solutions terminate in afinite-time singularity. The blow-up time is delayed by increasingthe curvature of the surface. These results are corroboratedby asymptotic predictions valid in the limits of small and largeamplitude and frequency. For large Reynolds number, the problemreduces to the two-dimensional stagnation-point flow againsta plane wall studied by previous authors. 相似文献
2.
Marino Cavazza Gioia Morganti Antonio Guerriero Francesco Pietra 《Tetrahedron letters》1980,21(38):3703-3704
EtO? in DMSO adds to ethylthiopicrate at C-3 to generate an ephemeral -adduct which ends into 1-ethylthio-4-ethoxy-2,5-dinitrobenzene, whilst ethyl picrate gives two adducts of attack of EtS? at C-3 or C-1. 相似文献
3.
Zampella G Fantucci P Pecoraro VL De Gioia L 《Journal of the American Chemical Society》2005,127(3):953-960
Density functional theory has been used to investigate structural, electronic and reactivity properties of complexes related to the peroxo forms of vanadium haloperoxidases (VHPO). In particular, the reactivity of the cofactor as a function of protonation state and environment, which are two factors thought to be crucial in modulating the activity of the enzyme, has been examined. In full agreement with experimental data, results highlight the role of protonation in the activation of the peroxo-vanadium complexes and show that the oxo-transfer step involves the unprotonated axial peroxo oxygen atom, which is easily accessible to substrates in the peroxo form of the enzyme. The role of Lys353, which in the X-ray structure of the peroxide-bound form of vanadium chloroperoxidase is hydrogen bonded to the equatorial oxygen atom of the peroxo group, has been also explored. It is concluded that Lys353 can play a role similar to a H+ in the activation of the peroxo form of the cofactor. 相似文献
4.
Hydrogenases catalyze the reversible oxidation of dihydrogen to protons and electrons. The structures of two Fe-only hydrogenases have been recently reported [Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science 1998, 282, 1853-1858. Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, E. C.; Fontecilla-Camps, J. C. Structure 1999, 7, 13-23], showing that the likely site of dihydrogen activation is the so-called [2Fe](H) cluster, where each Fe ion is coordinated by CO and CN(-) ligands and the two metals are bridged by a chelating S-X(3)-S ligand. Moreover, the presence of a water molecule coordinated to the distal Fe2 center suggested that the Fe2 atom could be a suitable site for binding and activation of H(2). In this contribution, we report a density functional theory investigation of the structural and electronic properties of complexes derived from the [(CO)(CH(3)S)(CN)Fe(II)(mu-PDT)Fe(II)(CO)(2)(CN)](-1) species, which is related to the [2Fe](H) cluster observed in Fe-only hydrogenases. Our results show that the structure of the [2Fe](H) cluster observed in the enzyme does not correspond to a stable form of the isolated cluster, in the absence of the protein. As a consequence, the reactivity of [(CO)(CH(3)S)(CN)Fe(II)(mu-PDT)Fe(II)(CO)(2)(CN)](-1) derivatives in solution may be expected to be quite different from that of the active site of Fe-only hydrogenases. In fact, the most favorable path for H(2) activation involves the two metal atoms and one of the bridging S atoms and is associated with a very low activation energy (5.3 kcal mol(-1)). The relevance of these observations for the catalytic properties of Fe-only hydrogenases is discussed in light of available experimental and theoretical data. 相似文献
5.
Density functional theory has been used to investigate complexes related to the [2Fe](H) subcluster of [Fe]-hydrogenases. In particular, the effects on structural and electronic properties of redox state and ligands with different sigma-donor pi-acceptor character, which replace the cysteine residue coordinated to the [2Fe](H) subcluster in the enzyme, have been investigated. Results show that the structural and electronic properties of fully reduced Fe(I)Fe(I) complexes are strongly affected by the nature of the ligand L, and in particular, a progressive rotation of the Fe(d)(CO)(2)(CN) group, with a CO ligand moving from a terminal to a semibridged position, is observed going from the softest to the hardest ligand. For the partially oxidized Fe(I)Fe(II) complexes, two isomers of similar stability, characterized either by a CO ligand in a terminal or bridged position, have been observed. The switching between the two forms is associated with a spin and charge transfer between the two iron atoms, a feature that could be relevant in the catalytic mechanism of dihydrogen activation. The structure of the fully oxidized Fe(II)Fe(II) models is extremely dependent on the nature of the L ligand; one CO group coordinated to Fe(d) switches from terminal to bridging position going from complexes characterized by neutral to anionic L ligands. 相似文献
6.
Bartoli G Bartolacci M Bosco M Foglia G Giuliani A Marcantoni E Sambri L Torregiani E 《The Journal of organic chemistry》2003,68(11):4594-4597
Alkylation of indoles by means of the Michael addition has been the subject of a number of investigation. It is well established that regioselectivity in the additions of indoles to electron-deficient alkenes is strongly controlled by the reaction medium. In a continuation of the work on developing greener and cleaner technologies, the cerium(III) chloride heptahydrate and sodium iodide combination supported on silica gel catalyzes the alkylation of various indoles with alpha,beta-unsaturated ketones giving 3-(3-oxoalkyl)indole derivatives in good yields. The substitution on the indole nucleus occurred exclusively at the 3-position, and N-alkylation products have not been observed. 相似文献
7.
G. Gioia Lobbia G. Berchiesi M. A. Berchiesi 《Journal of Thermal Analysis and Calorimetry》1977,12(3):449-454
The liquid-liquid equilibrium regions in binary systems, one component of which is a dicarboxylic aliphatic acid, are examined by the linear equation in (x
1/x1)=a+bT. Theb term is plottedvs. the difference of polar character and one graph is obtained for the different systems.We thank the CNR (Rome) for financial aid. 相似文献
8.
G. Berchiesi G. Gioia Lobbia M. A. Berchiesi G. Vitali 《Journal of Thermal Analysis and Calorimetry》1984,29(4):729-732
This paper presents a final discussion on studies of solutions of electrolytes in molten acetamide. These studies deal with cryoscopic and ultrasonic (by shear waves) techniques and reveal the complexity of the behaviour of these systems due to strong ion-solvent interactions.
Thanks are due to CNR and MPI (Rome) for financial support, and to R. Borgarucci for technical assistance. 相似文献
Zusammenfassung Die über Lösungen von Elektrolyten in geschmolzenem Acetamid ausgeführten Untersuchungen werden diskutiert. Diese Untersuchungen erstrecken sich auf kryoskopische und Ultraschall-Methoden und vermitteln ein Bild von der durch starke Ionen-Lösungsmittel-Wechselwirkungen bedingten Komplexität des Verhaltens dieser Systeme.
. , - - .
Thanks are due to CNR and MPI (Rome) for financial support, and to R. Borgarucci for technical assistance. 相似文献
9.
Casella L Monzani E Fantucci P Gullotti M De Gioia L Strini A Chillemi F 《Inorganic chemistry》1996,35(2):439-444
The effect of strain in the axial coordination of imidazole to the heme has been studied in the chelate complexes deuterohemin-histidine (DH-His) and deuterohemin-alanylhistidine (DH-AlaHis). Molecular mechanics calculations indicate that three types of distortion of the axial ligand occur in DH-His, due to the relatively short length of the arm carrying the donor group: tilting off-axis, tipping, and inclination of the imidazole plane with respect to the axial Fe-N bond. The effects of tilting (Deltagamma approximately 10 degrees ) and inclination of the imidazole ring (Deltadelta approximately 17 degrees ) are dominant, while tipping is small and is probably of little importance here. By contrast, the axial imidazole coordination is normal in DH-AlaHis and other computed deuterohemin-dipeptide or -tripeptide complexes where histidine is the terminal residue, the only exception being DH-ProHis, where the rigidity of the proline ring reduces the flexibility of the chelating arm. The distortion in the axial iron-imidazole bond in DH-His has profound and negative influence on the binding and catalytic properties of this complex compared to DH-AlaHis. The former complex binds more weakly carbon monoxide, in its reduced form, and imidazole, in its oxidized form, than the latter. The catalytic efficiency in peroxidative oxidations is also reduced in DH-His with respect to DH-AlaHis. The activity of the latter complex is similar to that of microperoxidase-11, the peptide fragment incorporating the heme that results from hydrolytic cleavage of cytochrome c. 相似文献
10.
Monia Vadrucci Anna Mazzinghi Beatrice Sorrentino Stella Falzone Claudia Gioia Patrizia Gioia Ersilia M. Loreti Massimo Chiari 《X射线光谱测定》2020,49(6):668-678
Scientific investigation is very important in studies addressing issues of archaeological and historical objects. Ion beam analysis (IBA) and macro X-ray fluorescence (MA-XRF) spectroscopy are remarkable tools to obtain information about elemental composition and imaging of historical artefacts with a non-invasive character. These investigation techniques were employed in the framework of a project aimed at supporting the characterization of materials and techniques related with the Roman wall painting. The archaeological excavations at Villa della Piscina in Rome have revealed a luxury building with a large pool (about 50 m long) and thermal baths and numerous fragments of plaster, coming from intentional demolitions referable to two distinct architectural contexts of the Villa during the imperial age, have been found. This work deals with studying the interesting wall pictorial apparatus of great cultural value of the heritage inherited from the Roman age in the area of the ancient city of Rome. The colour palette of the pigments investigated, in particular, by MA-XRF and particle-induced X-ray emission (PIXE) and has revealed Fe, Cu, Pb, Si and Hg as main elements. Traces of other constituents uncover the choice of the different colours chosen by the artists who had embellished the rooms of the Villa. 相似文献